cada2bcb03
merge_all/_merged/index.json is now a shard manifest; activities are
split into index-{year}.json files. The feed loads only the most-recent
year on first paint (~200 activities instead of all of them). Older
years are fetched lazily when the user clicks "Load older activities".
Also strips best_efforts / best_climb_m / source from shard files —
these fields are aggregation inputs only, never read by the feed UI.